Add-ins are 3rd party extensions that can add features or enhance Outlook.com functionality. So, when composing emails in Outlook, extra add-ins like Evernote, Bing Maps, scheduled meetings etc are enabled and loaded by default.
These however can be turned off anytime if needed.
Here is how :
Sign into Outlook and choose Options from the gear icon drop down menu.
Scroll to General > Manage add-ins. This will show a list of active add-ins. Disable them individually as required.
Once disabled, these add-ins will no longer be visible or loaded when composing/replying to emails.